As is well understood by those of skill in the art, the transferring of a subscriber station from one base station to another is termed handoff. In simple terms, handoff occurs when a subscriber station has to be handed off from one cell to another as the subscriber station moves between cells. Hard handoff refers to breaking the connection in a current cell and then making a new connection in the new cell. Hard handoff is also referred to as a “break-before-make” handoff. While Code Division Multiple Access (“CDMA”) offers the opportunity to perform a “make-before-break” or “soft” handoff when adjacent CDMA cells all the same frequency, situations still occur in a CDMA system when a subscriber station will transition between cells that operate on different frequencies. Thus, CDMA systems that span multiple frequencies still require equipment to effect hard handoff.
There are several known methods of effecting hard handoff. One common method is by means of a round trip delay (“RTD”) trigger. In simple terms, the RTD can be used to establish a distance between the subscriber station and the corresponding base station. Where the established distance indicates that the subscriber station is at the frequency edge, hard handoff is triggered. While RTD trigger is perhaps the most common method of effecting hard handoff it is not always reliable, and thus it is also known to use a pilot beacon trigger to effect hard handoff. Typically, a pilot beacon trigger can be used in both idle and traffic mode transitions, whereas RTD is used in traffic mode transitions only. When using a pilot beacon trigger, a radio frequency signal is transmitted on a pilot channel, that forces the subscriber station to transition to another frequency. Pilot beacon triggers can be effected with a pilot beacon unit, such as the Ericsson/Qualcomm QCPlus Pilot Beacon. Unfortunately, the QCPlus Pilot Beacon has been discontinued, and so it can be difficult to readily obtain a pilot beacon unit for to effect pilot beacon triggers. One way of effecting a pilot beacon trigger hard handoff without a pilot beacon unit is to use a channel element card in the base station radio that is enabled to transmit the appropriate radio frequency signal to force the subscriber station to transition to another frequency on hard handoff. In general, either of these solutions tend to be costly in a multi-carrier environment, such as that found in an urban centre like the Greater Toronto Area in Ontario, Canada.
Another method for effecting hard handoff is using a so-called Enhanced Hard Handoff, which is a hard handoff based on forward and reverse call statistic parameters such as frame-error-rate (“FER”) or a received signal to noise ratio expressed as Eb/No. However, it is believed that pilot beacon triggered HHO mechanisms have proved to be generally reliable, whereas EHHO can be difficult to implement in the field, particularly in relation to pilot beacon triggered HHO.